The use of semipermeable membranes for effecting liquid separations has become well accepted, and membranes both for ultrafiltration applications and reverse-osmosis applications are presently in use for a wide spectrum of applications, including water purification, concentration of dilute mixtures or solutions, and waste treatment and/or recovery of recyclable components. A large variety of semipermeable membranes have been developed, and spiral wound cartridges have been one accepted type of device which has been found valuable for efficiently providing a large amount of membrane surface area within a given spatial volume.
Spiral wound cartridges have been manufactured for years. While much progress and many improvements have been made, one particularly troublesome problem remains. The cartridges include various leaves attached to a permeate collection tube. One leaf is a sandwich assembly including a length of a semipermeable membrane sheet folded in half with the fold adjacent the tube, and with a length of a feed carrier material positioned between the sheet halves. The problem is the formation with time of cracks or other degradation of the membrane along the fold line. Such degradation can permit some of the feed liquid to enter the permeate collection tube.
The semipermeable membrane sheet can be of the asymmetric type, such as the cellulose acetate membranes disclosed in U.S. Pat. Nos. 3,133,137 and 3,344,214 wherein a thin, active, dense layer is formed at one surface of cast polymeric material by selective evaporation or the like, whereas the remainder of the membrane throughout and extending to the other surface is of a much more porous composition which tends to integrally support the dense active surface layer which exhibits the semipermeable characteristics. With the asymmetric membrane used at low pressure the problem was generally not serious. However, when used at higher pressures (over 400 psi) and for applications with severe use and cleaning conditions, for example, in the dairy industry, the fold problem can become pronounced.
More recently developed is the composite or thin film membrane sheet. In this type membrane, a dense, active layer is formed of a chemically different material than the nonactive supporting layer, examples of these being shown in U.S. Pat. Nos. 4,259,183 and 4,265,745. Such composite membranes can be made by any suitable method; however, frequently an interfacial condensation reaction is carried out whereby a thin film is formed by reactants which create a thin, dense, polymeric surface, such as a polyamide having the desired semipermeable characteristics. The porous, less dense, supporting layer adjacent which the interfacial condensation reaction takes place can be of any suitable polymeric material, such as a polysulfone, having the desired pore size to adequately support the ultra-thin, interfacial layer without creating undesirably high pressure drops across it. This type membrane supporting layer is cast upon a thin layer of polyester non-woven felt backing material which essentially integrally supports the membrane material against puncture and excessive deformation even at high feed pressures. Such a polyester felt backing layer is considered as being part of the membrane sheet itself, lying adjacent to the membrane nonactive surface and being sealed thereto along its outer edges by a flexible adhesive.
These newer composite membrane sheets have increased deficiencies in the fold area because of buckling of the membrane at the inside of the fold. The attempted solutions included use of a tape such as polypropylene either on the high pressure side (membrane side) or the low pressure side (felt backing side). Neither is particularly satisfactory. Tape on the high pressure or membrane side tends to come off during use or cleaning exposing the fold to potential crack leakage. When the tape is used on felt, the tape is held in place by pressure, but the fold is exposed directly to the high pressure feed. Any cracks developing will cause a leak which then allows the feed liquid to migrate through the felt under the tape until it clears the tape. Tape on felt is of some value but does not eliminate the problem.